Laminates bonded with adhesive of aqueous emulsion of interpolymer of vinyl acetate,n-methylol acrylamide and acrylic acid

ABSTRACT

1. A LAMINATE COMPRISING AT LEAST TWO SUPERIMPOSED LAYERS OF SOLID MATERIAL BONDED TOGETHER BY A CURED ADHESIVE COMPRISING AN AQUEOUS EMULSION OF A THERMOSETTABLE INTERPOLYMER OF ADDITIVELY COMBINED UNITS OF VINYL ACETATE, N-METHYLOL ACRYLAMIDE, AND ACRYLIC ACID, SAID INTERPOLYMER CONTAINING ABOUT 5 TO 20 WEIGHT PERCENT OF POLYMERIZED N-METHYLOL ACRYLAMIDE, ABOUT 1 TO 3 WEIGHT PERCENT OF POLYMERIZED ACRYLIC ACID, AND A BALANCE CONSISTING ESSENTIALLY OF POLYMERIZED VINYL ACETATE, SAID EMULSION CONTAINING ABOUT 40 TO 60 WEIGHT PERCENT OF WATER, AND SAID EMULSION BEING SUBSTANTIALLY DEVOID OF SOLID PARTICLES HAVING A SIZE LARGER THAN 150 MESH (U.S. SIEVE SERIES).

US. Cl. 161-251 11 Claims ABSTRACT OF THE DISCLOSURE A laminate, for instance, of wood, bonded with an adhesive comprising a grit-free, aqueous polymer emulsion, prepared by aqueous dispersion polymerization of vinyl acetate, N-methylol acrylamide, and acrylic acid in amounts sufficient to provide a thermosettable interpolymer containing about to 20 weight percent of N- methylol acrylamide, about 1 to 3 Weight percent of acrylic acid, and a balance consisting essentially of vinyl acetate.

This is a division of application Ser. No. 139,828, filed May 3, 1971, now Pat. No. 3,770,680.

This invention relates to a grit-free, aqueous emulsion of a thermosettable interpolymer, to methods of making the emulsion, and to products prepared from the emulsion. More particularly, it concerns a grit-free, aqueous emulsion of a thermosettable interpolymer of vinyl acetate, N-methylol acrylamide, and acrylic acid, to methods of making same, to adhesive compositions formulated from the emulsion, and to laminates prepared by using the adhesives.

The preparation of aqueous emulsions of thermosettable interpolymers of vinyl acetate and N-methylol acrylamide has often yielded gritty products, i.e., emulsions which contain solid particles that are sufiiciently large as to be retained on a 150 mesh screen (U.S. Sieve Series). This grittiness renders the emulsions unsuitable for most coating or adhesive applications, and various methods of overcoming the problem have been proposed. Thus, in US. Pats. Nos. 3,288,740 and 3,301,809 aqueous emulsions of interpolymers of vinyl acetate and N-methylol acrylamide are prepared by polymerizing the monomers while in admixture either with an alkaline earth metal salt of an alpha, beta-unsaturated monocarboxylic acid or with polyvinyl alcohol.

It has now been discovered that a grit-free emulsion of a thermosettable interpolymer of vinyl acetate and N methylol acrylamide can also be obtained by including acrylic acid in the polymerization mixture in an amount sufiicient to provide about 1 to 3 weight percent of polymerized acrylic acid in the resultant interpolymer product. Moreover, this smooth emulsion product can be obtained even though the interpolymer is substantially devoid of either or both of polyvinyl alcohol and alkaline earth metal salt of alpha, beta-unsaturated monocarboxylic acid. The polymer emulsion of this invention is an excellent base composition for aqueous emulsion adhesives, most particularly for adhesives which are useful in plywood manufacture, as will hereinafter be discussed in more detail.

The grit-free polymer emulsion of this invention can be readily prepared by 1) forming an aqueous dispersion of (a) catalytically effective amounts of a suitable redox catalyst system, (b) about 5 to 20 weight percent of vinyl acetate monomer, and (c) a balance consisting essentially of water; (2) agitating the dispersion and United States Patent 0 maintaining it at a pH of about 4 to 6 and a temperature of about room temperature to 100 C., so as to initiate and sustain addition polymerization of the vinyl acetate; (3) maintaining the vinyl acetate monomer concentration in the dispersion at about 5 to 20 weight percent for about the first 5l5 minutes after initiation of the polym erization and thereafter maintaining it at about 1 /2 to 5 weight percent until all of the vinyl acetate has been added to the dispersion; and (4) adding to the dispersion during the polymerization about 6 to 15 parts of N- methylol acrylamide per each 100 parts by weight of total monomers employed, and about 1 to 3 parts of acrylic acid per each 100 parts by weight of total monomers employed.

Preferred conditions for the polymerization include a reaction temperature of about 40 to C.; an initial vinyl acetate concentration of about 5 to 15, most preferably about 5 to 7, weight percent; and about 6 to 10 parts of N-methylol acrylamide added per part of vinyl acetate.

As stated above, the catalyst system is of the redox type, for example a mixture of t-butyl hydroperoxide (as the oxidizing agent, also called the initiator) and sodium bisulfite (as the reducing agent, also called the activator). It is also preferred to have trace amounts of ferrous ion present in the recation mixture in order to initate the polymerization. As a suitable source of ferrous ion may be mentioned, for example, ferrous ammonium sulfate.

The addition of the N-methylol acrylamide and acrylic acid to the polymerization dispersion is preferably effected incrementally and uniformly over the period during which the balance of the vinyl acetate is added, often about 1 to 5 or more hours. Polymerization is preferably continued after all of the monomers have been added and until the vinyl acetate monomer concentration drops to about 0.5 weight percent or lower. The overall time of the polymerization reaction will usually be about 1 to 6, 7 or more hours.

The water content of the dispersion is preferably maintained between about 40 weight percent and about 60 weight percent during the polymerization, and the overall ratio of monomers employed is approximately the same as desired in the interpolymer product.

The pH of the polymerization mixture is maintained at about 4 to 6 by the inclusion of a suitable alkaline buifering agent in the reaction mixture, such as, for example, disodium phosphate, sodium acetate, or sodium benzoate. The amount of buffering agent is generally about .8 to 1.2 percent, based on the weight of monomers. The emulsion product will have a longer shelf life of the pH is 5 or more. If desired, then, the pH of the final emulsion can be elevated slightly by the addition of a base such as ammonium hydroxide.

The product resulting from the polymerization method of the present invention is a grit-free, aqueous emulsion of a thermosettable interpolymer of additively combined units of vinyl acetate, N-methylol acrylamide, and acrylic acid. wherein the interpolymer contains about 5 to 20, preferably about 7 to 15, weight percent of polymerized N-methylol acrylamide, about 1 to 3 weight percent of polvmerized acrylic acid, and a balance consisting essentially of polymerized vinyl acetate. The emulsion resulting from the polymerization reaction generally contains about 40 to 60 weight percent of water and has a viscosity, if adjusted to a water content of 50 weight percent, of about 300 to 2500 centipoises, as determined at 25 C. with a Brookfield viscometer using a No. 4 spindle at 6 revolutions per minute.

Depending on the intended use for the emulsion, it may be desired to add one or more thickening agents to Patented Oct. 15, 1974 it. If it is to be used as a base composition for formulating a wood adhesive, for example, it will generally be preferred that it have a viscosity of about 2000' to 7000, most advantageously about 4000 to 6000, centipoises, determined in the manner described above at 50 percent concentration. Suitable thickening agents for use with the emulsion of this invention include, for example, the cellulosic thickeners, e.g. methyl cellulose and hydroxyethyl cellulose. When formulating a wood adhesive, the amount of such a thickening agent that will usually be required to reach the desired viscosity will be about 0.5 to 1.5 percent, based on the weight of solids in the emulsion.

Formulation of an adhesive composition from the emulsion of the present invention is readily achieved by adding to the emulsion an effective amount of an acidic curing catalyst for the interpolymer. As examples of suitable curing catalysts may be mentioned chromium nitrate, aluminum chloride, phosphoric acid, chcromic acid, oxalic acid, nitric acid, and citric acid. The amount of catalyst employed will generally be that which is sufiicient to adjust the pH of the emulsion to about 1 to 3. This will usually require a minor amount, e.g. about 1 to percent, based on the weight of solids in the composition.

Curing of the adhesive compositions of the present invention will generally begin to take place at room temperature as soon as the catalyst and polymer emulsion have been intimately admixed, and the pot life of the compositions is generally no longer than about 3 days. Thus, the formulation should not be prepared except at or near the intended time of use.

The curing of the adhesive is best effected at elevated temperatures and with the work pieces held together under pressure. The adhesives of this invention are excellent for use in preparing laminates containing two or more superposed layers of solid material, e.g. wood, paper or fabric, and are especially suitable for manufacturing water-resistant plywood. In the latter application, suggested cure times will vary with pH, choice of catalyst, viscosity, etc., but it is generally advantageous to cure the adhesive at about 225 F. to 300 F. for about 3 to minutes.

EXAMPLE The following ingredients are charged to a 10 gallon, glass lined, Pfaudler kettle:

G. Vinyl acetate 900 Deionized H O 10,000

Disodium phosphate Ferrous ammonium sulfate (1 wt. percent aqueous solution) 4O Tertiarybutyl hydroperoxide 15 The resultant mixture, the pH of which is about 4.5, is heated to 60 C. under a nitrogen atmosphere. Polymerization is then initiated with 50 ml. of an activator solution consisting of:

G. Sodium bisulfite 300 Disodium phosphate 225 Deionized H O 3,000

Upon initiation there are continuously an simultaneously added subsurface to the mixture, over a two-hour period, two separate monomer charges consisting of:

Charge 1: G.

N-methylol acrylamide (60 wt. percent aqueous solution) 2750 Deionized H O 1928 4 Charge 2: G. Vinyl acetate 14,596 Acrylic acid 166 Tertiary butyl hydroperoxide (70' wt. percent pure; balance consisting of other peroxides and diperoxides) 46 The reaction temperature is maintained between 60- 65 C. with an average cooling jacket temperature of 36 C. Additional activator solution is charged to the kettle to maintain the polymerization and the vinyl acetate free monomer level is kept between 3 and 4 wt. percent. Approximately 1900 ml. of activator solution has been consumed by the end of the reaction. The reaction mixture is cooled when the vinyl acetate free monomer level reaches 0.4 wt. percent. The final pH of the emulsion is 4.8. The emulsion is free of grit, rendering it ideally suited for use as a base for a wood adhesive. So as to lengthen the emulsions shelf life, the pH is raised to 5.0 by the addition of ammonium hydroxide to the emulsion.

The emulsion solids constitute about 54 wt. percent of the emulsion, and the viscosity (measured at 50% water concentration by a Brookfield viscometer at 25 C. using a No. 4 spindle at 6 r.p.m.) is about 360 centipoises (cps).

Samples of emulsions prepared as described above were supplied with a curing catalyst and subjected to pot-life and wood adhesive tests, the results of which are reported hereinafter in Tables I through III.

Referring now to Table I, the following explanations are pertinent:

All of the emulsions tested were substantially the same except for solids content. Thus, Pfaudler #ll, Pfaudler #14, and Pfaudler #20 each contained about 50 weight percent solids, while Emulsion No. 60-4003 contained about 55 weight percent solids and Emulsion No. J-72 contained about 52.8 weight percent solids.

Methocel is used to designate a 5 weight percent aque ous solution of Methocel HGD.G.S., a methyl cellulose of 50,000 cps. viscosity. Natrosol is used to designate a hydroxyethyl cellulose marketed under that trademark, and Cellosize HEC is used to designate another brand of hydroxyethyl cellulose thickening agent.

The aluminum chloride hydrate was added in the form of a 50 weight percent aqueous solution, and the reported parts by weight are with reference to the solution.

Initial pH is the pH of the emulsion after all of the additives had been incorporated in it.

With regard to Table II, the viscosities reported therein were determined at 25 C. with a Brookfield viscometer, using a No. 4 spindle.

The following explanations are with regard to Table III:

The test samples were 3 ply laminates of one-sixteenth inch thick birch veneer which had been prepared using the test emulsion as the adhesive and an open time of 3 minutes. Each laminate was cured for 15 minutes at 225 F. and 200 p.s.i. and then aged for two weeks at room temperature.

The cold water and boiling water shear strength tests were performed according to the provisions of paragraphs 5.3.2 and 5.3.3 of the US. Department of Commerces Commodity Standard CS 45-60. The cold water test involved immersion of the laminate for 48 hours in room temperature water, followed by subjection to three cycles of 8 hours in a F. oven and 16 hours in room temperature water. The boiling water test involved immersion in boiling water for 4 hours, followed by drying for 20 hours in a 150 F. oven, followed by reimmersion for 4 hours in boiling water. The sample was then permitted to cool to room temperature before testing.

Each sample was tested while wet for shear strength on an Instron tester at a cross head speed of 0.5 inch per minute, a chart speed of 5 inches per minute, and at a jaw span of 1% inches.

TABLE I Ingredients (parts by weight) Emulsion Cellosize Initial Run number number Emulsion Methocel Natrosol HEC AlCla-6HzO pH 1. Plaudler #ll. l 2. l 2. Pfaudler #14 10 2. 2 3 Pfaudler #K) 10 2. 1 4 6 003-.. 10 J-72 TABLE II Viscosities (centipoises) Initial After24 After 48 After 96 After 120 After 144 Aiter168 AiterZ hours, hours, hours, hours, hours, hours, weeks, Run number 6 r.p.m. 60 rpm. 6r.p.m. 6 r,p.m. 6 r.p.m. 6 r.p.m. 6 r.p.m. 6 r.p.m. 6 r.p.m.

7,300 2,280 14,500 39,000 45,000 53,000 58,000 Gelled. 5,100 3,000 29,000 000 Gelle 2,600 1,400 9,600 2 52,000 53, 500 67,000 Gelled.

TABLE STRENGTHS of the adhesive emulsion is substantially devoid of alkal1ne earth metal salt of alpha, beta-unsaturated monocar- Cold Water Test Boiling Water Test boxylic acid t Shea w ood t She a Ii/ 001i 6. The laminate of claim 1, wherein the adhesive emul- S reng 8.1 ure S reng' 2.1 UIB Run number (lo/sq, in.) (percent) (lb./sq. in.) (percent) 25 has PH P about 4 9 7. The laminate of claim 6, wherein the adhesive emul- 233 3g 382 g? sion has a viscosity, if adjusted to a water content of 50 s 75 228 100 weight percent, of about 300 to 7000 centipoises as deg 88 termined at 25 C. with a Brookfield viscometer using a I claim: 1. A laminate comprising at least two superimposed layers of solid material bonded together by a cured adhesive comprising an aqueous emulsion of a thermosettable interpolymer of additively combined units of vinyl acetate, N-methylol acrylamide, and acrylic acid, said interpolymer containing about 5 to 20 weight percent of polymerized N-methylol acrylamide, about 1 to 3 weight percent of polymerized acrylic acid, and a balance consisting essentially of polymerized vinyl acetate, said emulsion containing about to 60 weight percent of water, and said emulsion being substantially devoid of solid particles having a size larger than 150 mesh (U.S. Sieve Series).

2. The laminate of claim 1, wherein the solid material is wood.

3. The laminate of claim 1 wherein the interpolymer of the adhesive emulsion is substantially devoid of polyvinyl alcohol.

4. The laminate of claim 3, wherein the interpolymer of the adhesive emulsion is substantially devoid of alkaline earth metal salt of alpha, beta-unsaturated monocarboxylic acid.

5. The laminate of claim 1, wherein the interpolymer No. 4 spindle at 6 revolutions per minute.

8. The laminate of claim 6, wherein the interpolymer of the adhesive emulsion contains about 7 to 15 weight percent of polymerized N-methylol acrylamide.

9. The laminate of claim 8, wherein the adhesive emulsion has a viscosity, if adjusted to a water content of weight percent, about 4000 to 6000 centipoises as determined at 25 C. with a Brookfield viscometer using a No. 4 spindle at 6 revolutions per minute.

10. The laminate of claim 1, wherein the adhesive emulsion contains an effective amount of an acidic curing catalyst.

11. The laminate of claim 1, wherein a curing catalyst is added to the adhesive emulsion in a range from about 1 to 10 percent, based on the weight of solids in the emulsion.

References Cited UNITED STATES PATENTS 3,679,620 7/1972 Bcresniewicz 26029.6 TA

MARION E. MCCAMISH, Primary Examiner Us. 01. X.R. 

1. A LAMINATE COMPRISING AT LEAST TWO SUPERIMPOSED LAYERS OF SOLID MATERIAL BONDED TOGETHER BY A CURED ADHESIVE COMPRISING AN AQUEOUS EMULSION OF A THERMOSETTABLE INTERPOLYMER OF ADDITIVELY COMBINED UNITS OF VINYL ACETATE, N-METHYLOL ACRYLAMIDE, AND ACRYLIC ACID, SAID INTERPOLYMER CONTAINING ABOUT 5 TO 20 WEIGHT PERCENT OF POLYMERIZED N-METHYLOL ACRYLAMIDE, ABOUT 1 TO 3 WEIGHT PERCENT OF POLYMERIZED ACRYLIC ACID, AND A BALANCE CONSISTING ESSENTIALLY OF POLYMERIZED VINYL ACETATE, SAID EMULSION CONTAINING ABOUT 40 TO 60 WEIGHT PERCENT OF WATER, AND SAID EMULSION BEING SUBSTANTIALLY DEVOID OF SOLID PARTICLES HAVING A SIZE LARGER THAN 150 MESH (U.S. SIEVE SERIES). 